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Frank and Brian,
The MBT's are only used at the surface. When diving they should be
totally full of water and not used for depth control. When the tanks are
being flooded, all air gets expelled, even the air in the line. That's how
the water gets in the line. Once my tanks are full of water I leave the
vent valves open and never feel a difference in buoyancy while diving..
The Variable Ballast Tank (VBT or hard ballast tank) is what is
use to adjust buoyancy. That's designed with all ports closable so once
it's set, the bubble can't shrink or expand while your diving.
Frank,
It looks to me like your system will work since your lines is sloped
back. Any water caught in them should eventually settle back in
the tank, but you can't have any dips.. A three eights line may be a
little small though. It will take a long time to vent and it's harder
for air to travel up as the water runs back. Half inch would be
better. Water will be caught in the pipe above the closed vent valve
though. If it's low to the water line, it may be blown out by the pressure
in the MBT but if it's not, a quick burst of fill air through your T will surely
clear the line.
Still, I feel the better way to deal with the vents is to place the valve
right at the top of the tank and run linkage rather then tubing. But it's
not the only way. As long as you can prevent water from getting trapped in
the line or have a method to blow it out it will work.
Dan H.
----- Original Message -----
Sent: Monday, March 17, 2008 2:04
PM
Subject: Re: [PSUBS-MAILIST] MBT air
lock
Hi guys. Is the air lock problem on the ballast tanks due
to the line from tank to valve being too long? Or is it that the valve
has a length of line above it that holds water ?
I know that the line can't have any low spots in it, or even just run
level, but must have a continuous up-slope. I remember Gary saying his
original installation had a low/level spot that trapped a little water and
effectively plugged the line, but he fixed it by moving the valve higher. His
valves are located outside the tower, with the valve handles inside
and a seal on the valve handle shaft. It's a nice clean installation.
I am planning on running the lines into the sub, and using a ''T''
with air line to blow the tanks, and the line exiting the sub near the top of
the hull. This method has the same number of thru-hulls, but has more
valves. One on each thru-hull, and one on the ''T'' for the air line. ( three
valves for each tank.)
I think Gary's has a separate air line going to the tank for
blowing dry so basically one thru-hull for the air line and one for the valve
handle shaft.
My ballast tanks are several inches higher at the connection, and I'm
hoping that with the short length of the plumbing, I should be able to avoid
any blockage.
The hole in the bottom of the tanks is low and 3 inch diameter. I was
planning on using a 1/2 inch thru-hull for releasing the air. Does anyone
think that's too small ?
I'll try to put a sketch of the system up so you guys can comment on it.
Frank D.
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